Constant current device



Nov. 7, 1967 J. J. H. PARK 3,351,824

CONSTANT CURRENT DEVICE Filed June 2, 1964 C 0 k Q a I g I K) I l I 0 5 E VOLTAGE-Q 23 M 24 United States Patent 3,351,824 CONSTANT CURRENT DEVICE Jacob J. H. Park, Ottawa, Ontario, Canada, assignor to Northern Electric Company Limited, Montreal, Quebec, Canada Filed June 2, 1964, Ser. No. 371,902 Claims priority, application Canada, Apr. 28, 1964, 901,469 4 Claims. (Cl. 317-234) This invention relates to a constant current device, that is to say a device which, once a minimum voltage has been applied to it, will continue to pass substantially the same current regardless of any further increase in the applied voltage, until a final breakdown voltage is reached.

Such devices have many applications in the field of electronic circuitry. They serve to reduce unnecessary power consumption, as well as to improve the stability of the circuits. Constant current devices also find application'in various electrical systems and appliances.

In the past, it has been common to fabricate complicated networks of resistance, capacitance, inductance and vacuum tubes or transistors to achieve constant current characteristics. Such devices are not only complicated but wasteful of power, and it is the prime object of the present invention to provide a simple solid state device which will serve this function reliably and economically.

It is a further object of the invention to provide a constant current device which is simple to fabricate.

Yet another object is the provision of a construction which is inherently versatile in relation to the choice of voltage and current ranges over which the device will operate.

Many of the devices that have been developed in the past for constant current characteristics are uni-directional in nature. Their use has thus been limited to direct current applications.

It is still a further object of the present invention to provide a device which is bidirectional in nature and which can thus be used in alternating current circuits.

To this end the invention consists of a constant current device comprising an element of semiconducting material having broad end portions and a narrow neck portion interconnecting said end portions, all said portions being doped to the same conductivity type, a central area extending across the neck portion being less heavily doped than the remaining end areas of the element.

It is desirable that the electric field in the neck portion should be as uniform as possible, and, for this purpose, it is preferred to form the neck portion between a pair of substantially semi-circular (or at least part circular) edges. It is also advantageous to make each of the junctions between the central area and the end areas extend in an are which follows an equipotential line of an electric field extending between the end portions of the element.

Embodiments of the present invention are illustrated in the accompanying drawings by way of example. It is to be understood that the invention is not limited to the specific structural features illustrated, the broad scope of the invention being defined in the appended claims.

In the drawings:

FIGURE 1 is a plan view of a first device according to the invention;

FIGURE 2 is a side view of the device of FIGURE 1;

FIGURE 3 is a section on the line IIIIII in FIG- URE 1;

FIGURE 4 is a voltage-current diagram illustrating the characteristics of the device of FIGURES 1 to 3; and

FIGURE 5 is a fragmentary plan view of an alternative construction.

As seen in FIGURES 1 to 3, the active part of the device consists of a slice 10 of semiconductor material, that is a material such as silicon or germanium which has a high mobility of majority carriers (electrons or holes). The slice 10 is doped with one of the known suitable impurity substances which may be chosen either to make the slice N type or P type. The slice 10 is formed with a comparatively narrow neck portion 11 interconnecting a pair of broad end portions 12 on the surface of which gold or other suitable conducting terminals 13 have been formed. For convenience of handling the slice 10 may be mounted in an inert substrate 14 of a suitable insulating material. For example, if the slice 10 is made of N type material, the substrate 14 could be of P type ma terial, or vice-versa. The substrate 14 also acts as a heat sink.

The neck portion 11 is provided with a central area 15 which differs from the remainder of the slice 10 by virtue of being less heavily doped. During the diffusing of the doping substance into the slice 10, the central area 15 will be obscured earlier in the process than the remaining areas of the slice, so as to receive less doping material. In this way, the remaining areas of the slice 10 become more heavily doped but with the same conductivity type as the central area 15. For example, if an N type material is used for the device, the central area 15 will be doped N while the remaining areas will be doped N+. The central area 15 is thus rendered a relatively poor conductor.

The shaping of the neck portion 11 is important. Preferably, it is defined by a pair of semi-circular edges 16 and 17. These curved edges may extend (as illustrated in FIGURE 1) into straight line portions 18 and 19 "before intersecting the straight side edges of the slice 10. Alternatively, the straight portions 13 and 19 may be dispensed with. The device will then be somewhat narrower than shown in FIGURE 1, the circular edges 16 and 17 themselves intersecting the straight side edges. The junction lines 20 and 21 between the less heavily doped central area 15 and the more heavily doped end areas are also curved, these curves following the equipotential lines that will result in the neck portion 11 from the application of a voltage across the two terminals 13.

As will be explained below, the length of the central area 15 (dimension Y) may be varied at will. It follows that, although the central (less heavily doped) area 15 will necessarily lie in the general vicinity of and extend across the neck portion 11, these two parts are not necessarily coterminous. For example the area 15 may be shorter than the neck portions, or vice versa.

FIGURE 4 shows the current-voltage characteristics of the device.

At very low voltages the current climbs quickly to a value at which it remains substantially constant for further increases in voltage, until finally at the breakdown voltage avalanche current flows. The desirable features of this current-voltage curve will now be explained, together with the manner in which such features are achieved by the present device. A first desirable feature is that the initial section A of the curve should be steep so as to reach point B where the current levels oil at a comparatively low voltage. It is also desirable that the knee C where the curve becomes horizontal should be comparatively sharp, in other words that the curve should change rapidly from the steep upward slope A to the fiat portion D. Another desirable feature is that the fiat portion D should be as fiat as possible, that is with a slope which is close to Zero. Finally it is desirable that the value of the voltage E where break-down occurs should be high when expressed as a ratio of the voltage B.

Thus, these desirable characteristics can be summarized as follows:

(1) Flatness of the curve portion D combined with sharpness of the knee C.

(2) A large ratio of E to B.

(3) A low value of B.

It has been found experimentally that the present construction enables these desirable characteristics to be achieved simultaneously and to a degree not possible with prior devices.

The main criterion for achievement of the firs-t characteristic (flatness of curve portion D) is uniformity of electric field in the central area 15. Such uniformity is achieved in the present device by the use of semi-circular edges 16 and 17 combined with the equipotential curves 20 and 21 defining the ends of the central area 15. It has been found that the length of the dimension Y between the centres of the curves 20 and 21 determines the value of B. By making this distance short (much shorter than illustrated, if desired) a low value of B can be achieved. The remaining desirable characteristic, a high ratio of E to -B, can be obtained by adopting a high doping ratio between the highly doped end areas and the lightly doped central area 15. Certain materials, depending upon their resistivity, have inherently better E to B ratios. After the preferred material has been chosen this variable can be further controlled by the doping ratio.

It will be possible to construct two families of curves similar to the curve shown in FIGURE 4. Increase of the dimension Y will move points B and E to higher voltage values, with the ratio of E to B remaining substantially constant. The construction is thus readily moified for choice of a preferred voltage range.

The other family of curves can be generated by varying the dimension X, the width of the central area 15 at its narrowest point. Increasing the value of X will increase the current value at which the knee C appears; thus the portion D of the curve will be elevated. This latter statement assumes a constant thickness for the slice 10. It is the cross-section of the material at the narrowest point of the neck which truly determines the constant current value, rather than the width dimension alone.

In practice, a typical value of the dimension X would be of the order of 30 to 60 microns, compared with a typical value of 0.02 inch for the dimension Z, the width of the full slice 10. A typical thickness for the slice might be 0.007 inch. There is nothing especially critical to these values, and indeed they may be varied widely to achieve particular desired characteristics. It is one of the impor tant merits of the present construction that it lends itself so readily to a wide choice of current and voltage ranges. The figures given above are intended merely to indicate the general orders to magnitude involved and to make it 4 clear that the electric field in the neck portion 11 is high compared with that in the remainder of the slice 10.

While the symmetrical neck arrangement of FIGURES l to 3 is preferred, satisfactory results can be achieved with the arrangement shown in FIGURE 5, where the central area 22 of material less heavily doped than the end areas 23 is formed at a straight side edge 24 of the device, the neck being formed between this edge and a semicircular curved surface 25 in generally the same manner as before.

It has also been found that encapsulation of the device in an atmosphere of oxygen further improves the flatness of the portion D of the curve.

I claim:

1. A constant current device comprising an element of semi-conducting material having end portions and a neck portion narrower than said end portions and interconnecting said end portions, all said portions being doped to the same conductivity type, a central area extending across the neck portion, said central area being still of said same conductivity type, but less heavily doped than the remaining areas of the element, wherein each junction between the central area and the end areas extends in an arc, said are being coincident with an equipotential line of an electric field upon application of a potential between the end portions of the element.

2. A constant current device according to claim 1, wherein the neck portion is defined between a pair of substantially circular edges.

3. A constant current device according to claim 1, wherein the neck portion is defined between a straight edge and a substantially circular edge.

4. A constant current device comprising a slice of semi-conductor material having end portions and a neck portion narrower than said end portions and interconnecting said end portions, said neck portion being defined by a pair of substantially semi-circular edges, said slice being doped to a selected conductivity type and having a central area extending across the neck portion and still of said same conductivity type, .but less heavily doped than the remaining areas of the element, the junctions between the more and less heavily doped areas each extending in an arc across the neck portion from one edge thereof to the other, said are being coincident with an equipotential line of 'an electric field upon application of a potential between said end portions.

References Cited UNITED STATES PATENTS 2,648,805 8/1953 Spenke et al 317235 2,829,075 4/1958 Pankove 1481.5 2,954,486 9/1960 Doucette et al. 307-88.5

JOHN W. HUCKERT, Primary Examiner.

I. SHEWMAKER, Assistant Examiner. 

1. A CONSTANT CURRENT DEVICE COMPRISING AN ELEMENT OF SEMI-CONDUCTING MATERIAL HAVING END PORTIONS AND A NECK PORTION NARROWER THAN SAID END PORTIONS AND INTERCONNECTING SAID END PORTIONS, ALL SAID PORTIONS BEING DOPED TO THE SAME CONDUCTIVITY TYPE, A CENTRAL AREA EXTENDING ACROSS THE NECK PORTION, SAID CENTRAL AREA BEING STILL OF SAID SAME CONDUCTIVITY TYPE, BUT LESS HEAVILY DOPED THAN THE REMAINING AREAS OF THE ELEMENT, WHEREIN EACH JUNCTION BETWEEN THE CENTRAL AREA AND THE END AREAS EXTENDS IN AN ARC, SAID ARC BEING COINCIDENT WITH AN EQUIPOTENTIAL LINE OF AN ELECTRIC FIELD UPON APPLICATION OF A POTENTIAL BETWEEN THE END PORTIONS OF THE ELEMENT. 